Funding is requested for the purchase of a Molecular Dynamics PhosphorImager 425F system to be established as a shared instrument resource within St. Jude Children's Research Hospital. This system is designed for ultra-sensitive detection and quantitation of radioactivity, using special storage phosphor screens instead of x-ray film. It consists of a computer-controlled screen reader, a screen eraser, storage phosphor screens, a directly connected computer workstation for data analysis and archiving, and two printers for low and high resolution printouts of data. The specific advantages of the 425F imager that will enhance the research projects of our users are: (i) an increased sensitivity to radioactivity, permitting significantly, shorter exposure times, improved detection of very weak signals, use of less radioisotope (especially important for in vivo labeling experiments), and less radioactive waste; and (ii) a linear response to exposure over a seven-log range, permitting more precise quantitation of radioactivity. There are currently no equivalent systems in the institution or within the local biomedical research community. The PhosphorImager will be fully occupied by a user group of 28 grant supported investigators, and will facilitate the efficient progress of multiple laboratory research programs.